Hide more netgroup-related stuff behind ifdefs

Otherwise compiler complains about unused variables when building
without netgroup support (e.g. musl):

 ../src/polkitbackend/polkitbackendduktapeauthority.c: In function 'js_polkit_user_is_in_netgroup':
../src/polkitbackend/polkitbackendduktapeauthority.c:1184:15: error: variable 'netgroup' set but not used [-Werror=unused-but-set-variable]
 1184 |   const char *netgroup;
      |               ^~~~~~~~
../src/polkitbackend/polkitbackendduktapeauthority.c:1183:15: error: variable 'user' set but not used [-Werror=unused-but-set-variable]
 1183 |   const char *user;
      |               ^~~~
cc1: all warnings being treated as errors

 ../src/polkitbackend/polkitbackendinteractiveauthority.c: In function 'get_users_in_net_group':
../src/polkitbackend/polkitbackendinteractiveauthority.c:2279:16: error: unused variable 'name' [-Werror=unused-variable]
 2279 |   const gchar *name;
      |                ^~~~
cc1: all warnings being treated as errors

../src/polkit/polkitidentity.c: In function 'polkit_identity_new_for_gvariant':
../src/polkit/polkitidentity.c:349:19: error: unused variable 'name' [-Werror=unused-variable]
  349 |       const char *name;
      |                   ^~~~
../src/polkit/polkitidentity.c:348:17: error: unused variable 'v' [-Werror=unused-variable]
  348 |       GVariant *v;
      |                 ^
cc1: all warnings being treated as errors

src/polkit/polkitidentity.c

@@ -345,17 +345,10 @@ polkit_identity_new_for_gvariant (GVariant  *variant,
     }
   else if (g_strcmp0 (kind, "unix-netgroup") == 0)
     {
+#ifdef HAVE_SETNETGRENT
       GVariant *v;
       const char *name;
 
-#ifndef HAVE_SETNETGRENT
-      g_set_error (error,
-                   POLKIT_ERROR,
-                   POLKIT_ERROR_FAILED,
-                   "Netgroups are not available on this machine");
-      goto out;
-#else
-
       v = lookup_asv (details_gvariant, "name", G_VARIANT_TYPE_STRING, error);
       if (v == NULL)
         {
@@ -365,6 +358,12 @@ polkit_identity_new_for_gvariant (GVariant  *variant,
       name = g_variant_get_string (v, NULL);
       ret = polkit_unix_netgroup_new (name);
       g_variant_unref (v);
+#else
+      g_set_error (error,
+                   POLKIT_ERROR,
+                   POLKIT_ERROR_FAILED,
+                   "Netgroups are not available on this machine");
+      goto out;
 #endif
     }
   else

src/polkitbackend/polkitbackendduktapeauthority.c

@@ -1180,13 +1180,13 @@ js_polkit_spawn (duk_context *cx)
 static duk_ret_t
 js_polkit_user_is_in_netgroup (duk_context *cx)
 {
+  gboolean is_in_netgroup = FALSE;
+#ifdef HAVE_SETNETGRENT
   const char *user;
   const char *netgroup;
-  gboolean is_in_netgroup = FALSE;
 
   user = duk_require_string (cx, 0);
   netgroup = duk_require_string (cx, 1);
-#ifdef HAVE_SETNETGRENT
   if (innetgr (netgroup,
                NULL,  /* host */
                user,

src/polkitbackend/polkitbackendinteractiveauthority.c

@@ -2276,11 +2276,10 @@ static GList *
 get_users_in_net_group (PolkitIdentity                    *group,
                         gboolean                           include_root)
 {
+  GList *ret = NULL;
+#ifdef HAVE_SETNETGRENT
   const gchar *name;
-  GList *ret;
 
-  ret = NULL;
-#ifdef HAVE_SETNETGRENT
   name = polkit_unix_netgroup_get_name (POLKIT_UNIX_NETGROUP (group));
 
 # ifdef HAVE_SETNETGRENT_RETURN